Light receiving element, optical communication device, and method for manufacturing a light receiving element

ABSTRACT

A light receiving element ( 1 ) according to an embodiment of the disclosure includes a semiconductor layer ( 20 ) in which a photodiode having a PIN structure is provided in a mesa portion having a pillar shape. The photodiode includes a first conductive layer ( 21 ), an optical absorption layer ( 23 ), and a second conductive layer ( 24 ) having a light incident surface. In the light receiving element ( 1 ), the semiconductor layer ( 20 ) includes, in the vicinity of an interface between the first conductive layer ( 21 ) and the optical absorption layer ( 23 ), a constricted portion ( 26 ) that is the most constricted of the first conductive layer ( 21 ). The interface has an end exposed on an internal surface of the constricted portion ( 26 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of International ApplicationNo. PCT/JP2017/006257, filed Feb. 21, 2017, which claims priority toJapanese Application No. 2016-097799, filed May 16, 2016, thedisclosures of which are incorporated herein by reference.

BACKGROUND

The disclosure relates to a light receiving element, an opticalcommunication device, and a method for manufacturing a light receivingelement.

As a light receiving element for optical communication, a mesa-typeP-Intrinsic-N (PIN) photodiode is known, for example (See PTL1, forexample).

SUMMARY

A light receiving element for optical communication needs to balancemountability and high-speed responsiveness. It is desirable to provide alight receiving element, an optical communication device, and a methodfor manufacturing a light receiving element that are able to balancemountability and high-speed responsiveness.

A first light receiving element according to an embodiment of thedisclosure includes a semiconductor layer in which a photodiode isprovided in a mesa portion having a pillar shape. The photodiode has aPIN structure that includes a first conductive layer, an opticalabsorption layer, and a second conductive layer having a light incidentsurface. In the first light receiving element, the semiconductor layerincludes, in the vicinity of an interface between the first conductivelayer and the optical absorption layer, a constricted portion that isthe most constricted part of the first conductive layer. The interfacehas an edge exposed on an internal surface of the constricted portion.

A first optical communication device according to an embodiment of thedisclosure includes one or more light receiving elements. The one ormore light receiving elements each provided in the first opticalcommunication device has the same components as the first lightreceiving element as described above.

A method for manufacturing the first light receiving element accordingto the embodiment of the disclosure includes the following process:

(1) A process of etching a semiconductor layer into a shape of a mesa,in which the semiconductor layer includes a photodiode having a PINstructure that includes a first conductive layer, an optical absorptionlayer, and a second conductive layer, providing, in the vicinity of aninterface between the first conductive layer and the optical absorptionlayer in the mesa, a constricted portion that is the most constrictedpart of the first conductive layer, and exposing an edge of theinterface to the internal surface of the constricted portion.

In the first light receiving element, the first optical communicationdevice, and the method for manufacturing the first light receivingelement according to the embodiment of the disclosure, the mesa portionincludes, in the vicinity of the interface between the first conductivelayer and the optical absorption layer, a constricted portion that isthe most constricted part of the first conductive layer. The interfacehas an edge exposed on the internal surface of the constricted portion.This allows an element to have a smaller capacitance compared to thecase where the constricted portion is not provided in the mesa portion.In addition, even in the case of a larger mesa diameter, it is possibleto suppress an increase in the parasitic capacitance of the element.

A second light receiving element according to an embodiment of thedisclosure includes a semiconductor layer in which a photodiode isprovided in a mesa portion having a pillar shape. The photodiode has aPN structure that includes a first conductive layer and a secondconductive layer having a light incident surface. In the second lightreceiving element, the semiconductor layer includes, in the vicinity ofthe interface between the first conductive layer and the secondconductive layer, a constricted portion that is the most constrictedpart of the first conductive layer. The interface has an edge exposed onthe internal surface of the constricted portion.

The second optical communication device according to the embodiment ofthe disclosure includes one or more light receiving elements. The one ormore light receiving elements each provided in the second opticalcommunication device has the same components as those of the secondlight receiving element as described above.

A method for manufacturing the second light receiving element accordingto the embodiment of the disclosure includes the following process:

(1) A process of etching a semiconductor layer into a shape of a mesa,in which the semiconductor layer including a P-type photodiode thatincludes a first conductive layer and a second conductive layer,providing, in a vicinity of an interface between the first conductivelayer and the second conductive layer in the mesa, a constricted portionthat is the most constricted part of the first conductive layer, andexposing an edge of the interface to the internal surface of theconstricted portion.

In the second light receiving element, the second optical communicationdevice, and the method for manufacturing the second light receivingelement according to the embodiment of the disclosure, the mesa portionincludes, in the vicinity of an interface between the first conductivelayer and the second conductive layer, the constricted portion that isthe most constricted part of the first conductive layer. The interfacehas an edge exposed on the internal surface of the constricted portion.This allows an element to have a smaller capacitance compared to thecase where the constricted portion is not provided in the mesa portion.In addition, even in the case of a larger mesa diameter, it is possibleto suppress an increase in the parasitic capacitance of the element.

In the first light receiving element, the first optical communicationdevice, and the method for manufacturing the first light receivingelement according to the embodiment of the disclosure, the mesa portionincludes, in the vicinity of an interface between the first conductivelayer and the optical absorption layer, the constricted portion that isthe most constricted part of the first conductive layer. At the sametime, the interface has an edge exposed on the internal surface of theconstricted portion. This makes it possible to balance mountability andhigh-speed responsiveness.

In the second light receiving element, the second optical communicationdevice, and the method for manufacturing the second light receivingelement according to the embodiment of the disclosure, the mesa portionincludes, in the vicinity of an interface between the first conductivelayer and the second conductive layer, the constricted portion that isthe most constricted part of the the first conductive layer. At the sametime, the interface has an edge exposed on the internal surface of theconstricted portion. This makes it possible to balance mountability andhigh-speed responsiveness.

It is to be noted that the effects described here are not necessarilylimitative, and may have any of the effects described in the disclosure

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a cross-sectionalconfiguration of a light receiving element according to a firstembodiment of the disclosure.

FIG. 2 is an enlarged view illustrating an example of a cross-sectionalconfiguration in the vicinity of a constriction-formed layer in FIG. 1.

FIG. 3 is a diagram illustrating an example of a manufacturing processof the light receiving element in FIG. 1.

FIG. 4 is a diagram illustrating an example of a manufacturing processthat follows FIG. 3.

FIG. 5 is a diagram illustrating an example of a manufacturing processthat follows FIG. 4.

FIG. 6 is a diagram illustrating an example of a manufacturing processthat follows FIG. 5.

FIG. 7 is a diagram illustrating an example of a manufacturing processthat follows FIG. 6.

FIG. 8 is an enlarged view illustrating a modification example of thecross-sectional configuration in the vicinity of the constriction-formedlayer in FIG. 2.

FIG. 9 is an enlarged view illustrating a modification example of thecross-sectional configuration in the vicinity of the constriction-formedlayer in FIG. 2.

FIG. 10 is a diagram illustrating a modification example of thecross-sectional configuration of the light receiving element in FIG. 1.

FIG. 11 is a diagram illustrating an example of a manufacturing processof the light receiving element in FIG. 10.

FIG. 12 is a diagram illustrating an example of a manufacturing processthat follows FIG. 11.

FIG. 13 is a diagram illustrating an example of a manufacturing processthat follows FIG. 12.

FIG. 14 is a diagram illustrating an example of a manufacturing processthat follows FIG. 13.

FIG. 15 is a diagram illustrating an example of a manufacturing processthat follows FIG. 14.

FIG. 16 is a diagram illustrating an example of a cross-sectionalconfiguration of a light receiving element according to a secondembodiment of the disclosure.

FIG. 17 is an enlarged view illustrating an example of a cross-sectionalconfiguration in the vicinity of the constriction-formed layer in FIG.16.

FIG. 18 is a diagram illustrating an example of a manufacturing processof the light receiving element in FIG. 16.

FIG. 19 is a diagram illustrating an example of a manufacturing processthat follows FIG. 18.

FIG. 20 is a diagram illustrating an example of a manufacturing processthat follows FIG. 19.

FIG. 21 is a diagram illustrating an example of a manufacturing processthat follows FIG. 20.

FIG. 22 is an enlarged view illustrating a modification example of thecross-sectional configuration in the vicinity of the constriction-formedlayer in FIG. 17.

FIG. 23 is an enlarged view illustrating a modification example of thecross-sectional configuration in the vicinity of the constriction-formedlayer in FIG. 17.

FIG. 24 is a modification example of the cross-sectional configurationof the light receiving element in FIG. 16.

FIG. 25 is a diagram illustrating an example of a cross-sectionalconfiguration of an optical communication device according to a thirdembodiment of the disclosure.

DETAILED DESCRIPTION

In the following, some embodiments of the disclosure are described indetail with reference to the drawings. The following description is aspecific example of the disclosure, and the disclosure is not limited bythe following embodiments. In addition, the disclosure is not limited byposition, dimension, proportion, etc., either. It is to be noted thatdescriptions are given in the following order:

1. First Embodiment (light receiving element)

-   Example in which a constricted portion is provided in a light    receiving element of an upper surface incident type    2. Modification Examples of the First Embodiment (light receiving    element)-   Example in which the constriction-formed layer is omitted-   Example in which the constriction-formed layer includes more than    one layer-   Example in which the optical absorption layer is omitted-   Variation of etchant    3. Second Embodiment (light receiving element)-   Example in which a constricted portion is provided in a light    receiving element of a rear surface incident type    4. Modification Examples of the Second Embodiment (light receiving    element)-   Example in which the constriction-formed layer is omitted-   Example in which the constriction-formed layer includes more than    one layer-   Example in which the optical absorption layer is omitted-   Variation of etchant    5. Modification Examples of the Third Embodiment (optical    communication device)-   Example in which the light receiving element according to any of the    foregoing embodiments and modification examples is provided in the    optical communication device    1. First Embodiment    [Configuration]

A configuration of a light receiving element 1 according to a firstembodiment of the disclosure is described. FIG. 1 illustrates an exampleof a cross-sectional configuration of the light receiving element 1according to the present embodiment. The light receiving element 1 ispreferably applicable to an optical communication device that performssignal transmission (optical transmission) between semiconductor chipssuch as LSIs. The light receiving element 1 is a light receiving elementof an upper surface incident type, and includes, for example, asemiconductor layer 20 on a substrate 10. The substrate 10 includes, forexample, an undoped GaAs substrate. The semiconductor layer 20 has adevice structure in which, for example, a first conductive layer 21, anoptical absorption layer 23, and a second conductive layer 24 arelaminated in this order, starting from a side of the substrate 10.

The first conductive layer 21 (specifically, an upper portion of thefirst conductive layer 21), the optical absorption layer 23, and thesecond conductive layer 24 constitute a mesa portion 25 having a pillarshape. For example, the pillar-shaped mesa portion 25 has a shape of acylinder that extends in a normal direction of the substrate 10. Thepillar-shaped mesa portion 25 have a side surface that is parallel tothe normal direction of the substrate 10, or that has a forward taperedshape or a reverse tapered shape. An upper surface of the secondconductive layer 24 (a surface, of the second conductive layer 24,located on an opposite side of the optical absorption layer 23) isincluded in an upper surface of the mesa portion 25, and is a lightincident surface 20A through which light enters from outside.

Each of the first conductive layer 21 and the second conductive layer 24includes a semiconductor material of a conductivity type different fromeach other. The first conductive layer 21 includes, for example, n-typeAlGaAs. As an n-type impurity included in the n-type GaAs, silicon (Si)or selenium (Se) is given, for example. The second conductive layer 24includes, for example, p-type AlGaAs. As a p-type impurity included inthe p-type AlGaAs, zinc (Zn), magnesium (Mg), and beryllium (Be) aregiven.

The optical absorption layer 23 absorbs the light entering the lightincident surface 20A, and converts the absorbed light into an electricalsignal (photocurrent) corresponding to an output level of the absorbedlight. For example, the optical absorption layer 23 includes undopedGaAs or undoped InGaAs. There is provided, in the pillar-shaped mesaportion 25 in the semiconductor layer 20, a photodiode having a PINstructure that includes the first conductive layer 21 (specifically, theupper portion of the first conductive layer 21), the optical absorptionlayer 23, and the second conductive layer 24. In other words, the lightreceiving element 1 includes a mesa-type PIN photodiode. It is to benoted that the electrical signal generated in the optical absorptionlayer 23 is inputted, as an optical communication signal, into anoptical communication calculation circuit (not illustrated) coupled to afirst electrode 31 and a second electrode 32 that are described later.The optical communication signal is used in the optical communicationcalculation circuit for determining a signal level of the light that hasentered the light incident surface 20A.

The light receiving element 1 further includes, for example, the firstelectrode 31 and the second electrode 32. The first electrode 31 iselectrically coupled to the first conductive layer 21 and is provided,for example, in contact with a rim of the mesa portion 25 (specifically,a flat surface of the first conductive layer 21). For example, the firstelectrode 31 has a configuration in which an alloy of gold (Au) andgermanium (Ge), nickel (Ni), and gold (Au) are laminated in this order,starting from a side of the first conductive layer 21. The secondelectrode 32 is electrically coupled to the second conductive layer 24,and is provided, for example, in contact with an upper surface of thesecond conductive layer 24. For example, the second electrode 32 has anannular shape and has an opening. Of the upper surface of the secondconductive layer 24, a portion exposed to an inside of the opening ofthe second electrode 32 is the light incident surface 20A. The secondelectrode 32 may have an annular shape without a break, or may have anannular shape with a one or more breaks. The second electrode 32includes, for example, titanium (Ti), platinum (Pt), and gold (Au) thatare laminated in this order, starting from a side of the secondconductive layer 24.

Meanwhile, the semiconductor layer 20 includes, in the vicinity of theinterface 21A between the first conductive layer 21 and the opticalabsorption layer 23, a constricted portion 26 that is the mostconstricted part of the first conductive layer 21. For example, theconstricted portion 26 is rotationally symmetrical with respect to acentral axis of the mesa portion 25 as a rotational center (a linesegment parallel to an optical axis of the light receiving element 1 andpassing through a center of the mesa portion 25). The constrictedportion 26 is provided along an outer edge of the mesa portion 25,avoiding at least a part of a region opposing the light incident surface20A. In other words, the constricted portion 26 is provided in a mannernot to interrupt a current path in the semiconductor layer 20.

The constricted portion 26 has a wedge shape tapering toward the centralaxis of the mesa portion 25, with the interface 21A having an edgeexposed on an internal surface of the constricted portion 26. Theinterface 21A has a smaller area than the interface between the firstconductive layer 21 and the optical absorption layer 23 when assumingthat the constricted portion 26 is not provided. The narrowest part ofthe constricted portion 26 has a diameter (constriction diameter R1)smaller than a light reception diameter R2 of the light incident surface20A. The narrowest part of the constricted portion 26 has a depth(length) of, for example, not less than 2 μm and not more than 20 μm. Ina case where the constricted portion 26 has a length smaller than 2 μm,an effect produced by reducing the area of the interface 21A (effect ofreducing parasitic capacitance) is very small. In a case where theconstricted portion 26 has a length larger than 20 μm, a current path inthe semiconductor layer 20 for photoelectrons generated in the vicinityof the side surface of the mesa portion 25 within the optical absorptionlayer 23 is significantly larger than a current path in thesemiconductor layer 20 for photoelectrons generated in the middle of themesa portion 25 within the optical absorption layer 23. This is likelyto cause an adverse impact on high-speed responsiveness.

FIG. 2 is an enlarged view illustrating an example of a cross-sectionalconfiguration in the vicinity of the constricted portion 26. The firstconductive layer 21 includes a constriction-formed layer 22 at thenarrowest part of the constricted portion 26. The constriction-formedlayer 22 includes a material having a relatively high etching ratecompared to another part of the first conductive layer 21 except for theconstricted portion 26. In a case where the first conductive layer 21includes n-type AlGaAs, the constriction-formed layer 22 includes, forexample, n-type AlAs or n-type AlGaInP. When using, as an etchant, asolution in which phosphoric acid, hydrogen peroxide, and water aremixed at a ratio 3:1:50, AlAs has a higher etching rate than AlGaAs andGaAs. In addition, when using a mixed solution of hydrochloric acid andwater as the etchant, AlGaInP has a higher etching rate than AlGaAs andGaAs. In a case where the first conductive layer 21 has a thickness ofabout 2 μm and the optical absorption layer 23 has a thickness of about2 μm, the constriction-formed layer 22 has a thickness of not less than10 nm and not more than 100 nm, for example, and a thickness between theinterface 21A and the constriction-formed layer 22 within the firstconductive layer 21 is about 100 nm, for example.

The light receiving element 1 further includes, for example, insulativemembers (buried layer 27 and insulating film 28) that protect theconstricted portion 26. The buried layer 27 is provided to bury theconstricted portion 26, and includes, for example, a resin material suchas polyimide. The insulating film 28 is provided to cover a surface ofthe buried layer 27 and a surface of the mesa portion 25, and includesan insulative material. As the insulative material used for theinsulating film 28, SiO2 and SiN are given, for example.

[Manufacturing Method]

Next, a method for manufacturing the light receiving element 1 accordingto the present embodiment is described. FIG. 3 illustrates an example ofa manufacturing process of the light receiving element 1. FIG. 4illustrates an example of a manufacturing process that follows FIG. 3.FIG. 5 illustrates an example of a manufacturing process that followsFIG. 4. FIG. 6 illustrates an example of a manufacturing process thatfollows FIG. 5. FIG. 7 illustrates an example of a manufacturing processthat follows FIG. 6.

To manufacture the light receiving element 1, for example, a compoundsemiconductor is collectively provided on the substrate 10 that includesGaAs, using an epitaxial crystal growth method such as a metal organicchemical vapor deposition (MOCVD: Metal Organic Chemical VaporDeposition) method. At this time, for example, as a material for thecompound semiconductor, methyl-based organometallic gas such astrimethylaluminum (TMAl), trimethylgallium (TMGa), trimethylindium(TMIn), and arshin (AsH3) are used. As a material for donor impurity,hydrogen selenide (H2Se) is used, for example. As a material foracceptor impurity, dimethylzinc (DMZn) is used, for example.

It is to be noted that in FIG. 3, a first conductive layer 21D includesthe same material as the first conductive layer 21. Aconstriction-formed layer 22D includes the same material as theconstriction-formed layer 22. An optical absorption layer 23D includesthe same material as the optical absorption layer 23. A secondconductive layer 24D includes the same material as the second conductivelayer 24.

First, as illustrated in FIG. 3, for example, the first conductive layer21D (for example, n-type AlGaAs), the optical absorption layer 23D (forexample, non-doped GaAs), and the second conductive layer 24D (forexample, p-type AlGaAs) are provided on the substrate 10 in this order,starting from the side of the substrate 10. At the same time, theconstriction-formed layer 22D (for example, n-type AlAs or n-typeAlGaInP) is provided in the first conductive layer 21D. Next, forexample, a resist layer having a circular shape (not illustrated) isprovided, and thereafter the second conductive layer 24D and the opticalabsorption layer 23D are selectively etched using the resist layer as amask. At this time, it is preferable to use reactive ion etching (RIE)by Cl-based gas, for example. As illustrated in FIG. 4, for example,this provides, immediately under the resist layer, a mesa portion 25Dhaving a pillar shape with a side surface of each of the secondconductive layer 24 and the optical absorption layer 23 being exposedthereon.

Subsequently, the semiconductor layer that includes the first conductivelayer 21D, the constriction-formed layer 22D, the optical absorptionlayer 23, and the second conductive layer 24 is etched into a mesashape. At this time, wet etching is used. Here, in a case where theconstriction-formed layer 22D includes n-type AlAs, a solution in whichphosphoric acid, hydrogen peroxide, and water are mixed at a ratio3:1:50 is used as the etchant. In a case where the constriction-formedlayer 22D includes n-type AlGaInP, a solution in which hydrochloric acidand water is mixed is used as the etchant. Thus, as illustrated in FIG.5, for example, selective etching of the constriction-formed layer 22Dforms, in the vicinity of the interface 21A between the first conductivelayer 21D and the optical absorption layer 23 in the mesa (mesa portion25), the constricted portion 26 that is the most constricted part of thefirst conductive layer 21D, while exposing an edge of the interface 21Ato the internal surface of the constricted portion 26. As a result, themesa portion 25 including the constricted portion 26 in the side surfaceis provided.

Next, as illustrated in FIG. 6, for example, the buried layer 27 thatprotects the constricted portion 26 is provided. Subsequently, asillustrated in FIG. 7, for example, the insulating film 28, the firstelectrode 31, and the second electrode 32 are provided. Finally, forexample, the substrate 10 is polished and thereby thinned. In thismanner, the light receiving element 1 according to the presentembodiment is manufactured.

[Effects]

Next, effects of the light receiving element 1 according to the presentembodiment are described.

Generally, the light receiving element for communication needsmountability and high-speed responsiveness. However, mountability andhigh-speed responsiveness have a trade-off relationship. To increase adisplacement tolerance of a fiber, lens, etc. for easier mounting, it isdesirable to provide the largest possible light receiving area. Incontrast, to increase high-speed responsiveness, it is desirable toprovide the smallest possible parasitic capacitance of an element. For asmaller parasitic capacitance of the element, it is necessary to reducethe light receiving area (cross-sectional area of PIN junction).

A general light receiving element includes a ring electrode on the uppersurface of the mesa, and thus the cross-sectional area of the PINjunction is specified by the mesa diameter. For a smallercross-sectional area of the PIN junction, it is necessary to reduce themesa diameter. However, the smaller the mesa diameter becomes, thesmaller the area of an opening (the light receiving area) in the ringelectrode becomes.

In contrast, in the present embodiment, the mesa portion 25 includes, inthe vicinity of the interface 21A between the first conductive layer 21and the optical absorption layer 23, the constricted portion 26 that isthe most constricted part of the first conductive layer 21. Theinterface 21A has an edge exposed on the internal surface of theconstricted portion 26. This allows for a smaller parasitic capacitanceof the element as compared to the case of the mesa portion 25 withoutthe constricted portion 26. In addition, even in the case of a largermesa diameter, it is possible to suppress an increase in the parasiticcapacitance of the element. Thus, it is possible to balance mountabilityand high-speed responsiveness.

In addition, in the present embodiment, the constriction-formed layer 22is provided at the narrowest part of the constricted portion 26. Theconstriction-formed layer 22 includes a material having a relativelyhigh etching rate compared to another part of the first conductive layer21 except for the constricted portion 26. This makes it possible toeasily provide the constricted portion 26, utilizing selective etchingby wet etching. Thus, it is possible to balance mountability andhigh-speed responsiveness in a simple manner.

In addition, in the present embodiment, the narrowest part of theconstricted portion 26 has a diameter (constriction diameter R1) smallerthan the light reception diameter R2 of the light incident surface 20A.The narrowest part of the constricted portion 26 has a depth (length)of, for example, not less than 2 μm and not more than 20 μm. This makesit possible to balance mountability and high-speed responsiveness.

In addition, in the present embodiment, the insulative member (buriedlayer 27 or insulating film 28) is provided that protects theconstricted portion 26. This makes it possible to suppress deteriorationin reliability as a result of providing the constricted portion 26.

2. Modification Examples of the First Embodiment

Next, modification examples of the light receiving element 1 accordingto the foregoing embodiment are described.

[Modification Example A]

In the foregoing embodiment, as illustrated in FIG. 8, for example, theconstriction-formed layer 22 may be omitted, and the constricted portion26 may be provided in the vicinity of the interface 21A in thesemiconductor layer 20. However, in such a case, for example, theconstricted portion 26 may be provided in the following manner. Forexample, in the manufacturing process, a mask having an opening isprovided in a region where the constricted portion 26 is to be providedin a mesa-etched side surface of the first conductive layer 21 and theoptical absorption layer 23, and thereafter the constricted portion 26is formed by selectively etching the first conductive layer 21 and theoptical absorption layer 23 via the opening in the mask. In the presentmodification example, as with the foregoing embodiment, it is alsopossible to balance mountability and high-speed responsiveness.

[Modification Example B]

In any of the foregoing embodiment and modification examples thereof,the constriction-formed layer 22 may include more than one layer. Forexample, as illustrated in FIG. 9, the constriction-formed layer 22 maybe a laminate including two constriction-formed layers 22A and 22B. Atthis time, the constriction-formed layer 22A is provided closer to thesubstrate 10, and the constriction-formed layer 22B is provided closerto the optical absorption layer 23. For the same etchant, theconstriction-formed layer 22B has a higher etching rate than theconstriction-formed layer 22A. Thus, when the constriction-formed layers22A and 22B are etched using the same etchant, the constriction-formedlayer 22B is etched faster than the constriction-formed layer 22A, thusthe narrowest part of the constricted portion 26 is provided closer tothe optical absorption layer 23 in the constriction-formed layer 22B. Asa result, this makes it possible to easily provide the edge of theinterface 21A at a position closer to the center of the light receivingelement 1. Thus, it is possible not only to balance mountability andhigh-speed responsiveness, but also to easily achieve high-speedresponsiveness.

[Modification Example C]

In any of the foregoing embodiment and modification examples thereof,the optical absorption layer 23 may be omitted. At this time, forexample, as illustrated in FIG. 10, there is provided, in thepillar-shaped mesa portion 25 in the semiconductor layer 20, aphotodiode having a PN structure that includes the first conductivelayer 21 (specifically, the upper portion of the first conductive layer21) and the second conductive layer 24. In other words, in the presentmodification example, the light receiving element 1 includes a mesa-typePN photodiode.

In the present modification example, the semiconductor layer 20includes, in the vicinity of the interface 21A between the firstconductive layer 21 and the second conductive layer 24, the constrictedportion 26 that is the most constricted part of the first conductivelayer 21. The constricted portion 26 has a wedge shape tapering towardthe central axis of the mesa portion 25, and the interface 21A has anedge exposed on the internal surface of the constricted portion 26. Theinterface 21A has a smaller area than the interface between the firstconductive layer 21 and the second conductive layer 24 when assumingthat the constricted portion 26 is not provided. The narrowest part ofthe constricted portion 26 has a diameter (constriction diameter R1)smaller than the light reception diameter R2 of the light incidentsurface 20A. The narrowest part of the constricted portion 26 has adepth (length) of, for example, not less than 2 μm and not more than 20μm. In a case where the constricted portion 26 has a length smaller than2 μm, an effect produced by reducing the area of the interface 21A isvery small. In a case where the constricted portion 26 has a lengthlarger than 20 μm, within a depletion region that is provided in aregion including the interface 21A between the first conductive layer 21and the second conductive layer 24, a current path in the semiconductorlayer 20 for photoelectrons generated in the vicinity of the sidesurface of the mesa portion 25 is significantly longer than a currentpath in the semiconductor layer 20 for photoelectrons generated in themiddle of the mesa portion 25 within the depletion region. This islikely to have an adverse influence on high-speed responsiveness.

[Manufacturing Method]

Next, a method for manufacturing the light receiving element 1 accordingto the present modification example is described. FIG. 11 illustrates anexample of a manufacturing process of the light receiving element 1according to the present modification example. FIG. 12 illustrates anexample of a manufacturing process that follows FIG. 11. FIG. 13illustrates an example of a manufacturing process that follows FIG. 12.FIG. 14 illustrates an example of a manufacturing process that followsFIG. 13. FIG. 15 illustrates an example of a manufacturing process thatfollows FIG. 14.

First, as illustrated in FIG. 11, for example, the first conductivelayer 21D (n-type AlGaAs) and the second conductive layer 24D (p-typeAlGaAs) are provided on the substrate 10 in this order, starting fromthe side of the substrate 10. At the same time, the constriction-formedlayer 22D (n-type AlAs or n-type AlGaInP)) is provided in the firstconductive layer 21D. Next, for example, a resist layer having acircular shape (not illustrated) is provided, and thereafter the secondconductive layer 24D is selectively etched using the resist layer as amask. At this time, it is preferable to use RIE by Cl-based gas, forexample. As illustrated in FIG. 12, for example, this provides,immediately under the resist layer, the pillar-shaped mesa portion 25Dwith a side surface of the second conductive layer 24 being exposedthereon.

Subsequently, the semiconductor layer that includes the first conductivelayer 21D, the constriction-formed layer 22D, and the second conductivelayer 24 is etched into a mesa shape. At this time, wet etching is used.Here, in a case where the constriction-formed layer 22D includes n-typeAlAs, the solution in which phosphoric acid, hydrogen peroxide, andwater are mixed at a ratio 3:1:50 is used as the etchant. In a casewhere the constriction-formed layer 22D includes n-type AlGaInP, thesolution in which hydrochloric acid and water is mixed is used as theetchant. Thus, as illustrated in FIG. 13, for example, selective etchingof the constriction-formed layer 22D forms, in the vicinity of theinterface 21A between the first conductive layer 21D and the secondconductive layer 24 in the mesa (mesa portion 25), the constrictedportion 26 that is the most constricted part of the first conductivelayer 21D, while exposing an edge of the interface 21A to the internalsurface of the constricted portion 26. As a result, the mesa portion 25including the constricted portion 26 in the side surface is provided.

Next, as illustrated in FIG. 14, for example, the buried layer 27 thatprotects the constricted portion 26 is provided. Subsequently, asillustrated in FIG. 15, for example, the insulating film 28, the firstelectrode 31, and the second electrode 32 are provided. Finally, forexample, the substrate 10 is polished and thereby thinned. In thismanner, the light receiving element 1 according to the presentmodification example is manufactured.

In the present modification example, as with the foregoing embodiment,there is provided, in the vicinity of the interface 21A between firstconductive layer 21 and the second conductive layer 24, the constrictedportion 26 that is the most constricted part of the first conductivelayer 21. This makes it possible to balance mountability and high-speedresponsiveness.

In addition, in the present modification example, theconstriction-formed layer 22 is provided at the narrowest part of theconstricted portion 26. The constriction-formed layer 22 includes amaterial having a relatively high etching rate compared to another partof the first conductive layer 21 except for the constricted portion 26.This makes it possible to easily provide the constricted portion 26,utilizing selective etching by wet etching. Thus, it is possible tobalance mountability and high-speed responsiveness in a simple manner.

In addition, in the present modification example, the narrowest part ofthe constricted portion 26 has a diameter (constriction diameter R1)smaller than the light reception diameter R2 of the light incidentsurface 20A. The narrowest part of the constricted portion 26 has adepth (length) of, for example, not less than 2 μm and not more than 20μm. This makes it possible to balance mountability and high-speedresponsiveness.

In addition, in the present modification example, the insulative member(buried layer 27 or insulating film 28) that protects the constrictedportion 26 is provided. This makes it possible to suppress deteriorationin reliability as a result of providing the constricted portion 26.

[Modification Example D]

In any of the foregoing embodiment and modification examples thereof,the substrate 10 may include an InP substrate. In this case, the firstconductive layer 21 includes, for example, n-type InGaAsP, and thesecond conductive layer 24 includes, for example, p-type InGaAsP. In acase where the optical absorption layer 23 is provided, the opticalabsorption layer 23 includes, for example, non-doped InGaAs. In thiscase, the constriction-formed layer 22 includes, for example, n-typeInP. When using a mixed solution of phosphoric acid and hydrochloricacid as the etchant, InP has a higher etching rate than InGaAsP andInGaAs.

In the present modification example, the constriction-formed layer 22 isprovided at the narrowest part of the constricted portion 26. Theconstriction-formed layer 22 includes a material having a relativelyhigh etching rate compared to another part of the first conductive layer21 except for the constricted portion 26. This makes it possible toeasily provide the constricted portion 26, utilizing selective etchingby wet etching. Thus, it is possible to balance mountability andhigh-speed responsiveness in a simple manner.

[Modification Example E]

In any of the foregoing embodiment and Modification Examples A to D, thesubstrate 10 may include an InP substrate. In this case, the firstconductive layer 21 includes, for example, n-type InP, and the secondconductive layer 24 includes, for example, p-type InP. In a case wherethe optical absorption layer 23 is provided, the optical absorptionlayer 23 includes, for example, non-doped InGaAsP. In this case, theconstriction-formed layer 22 includes, for example, n-type InAlAs. Whenusing a mixed solution of sulfuric acid, hydrogen peroxide, and water asthe etchant, InAlAs has a higher etching rate than InP and InGaAs.

In the present modification example, the constriction-formed layer 22 isprovided at the narrowest part of the constricted portion 26. Theconstriction-formed layer 22 includes a material having a relativelyhigh etching rate compared to another part of the first conductive layer21 except for the constricted portion 26. This makes it possible toeasily provide the constricted portion 26, utilizing selective etchingby wet etching. Thus, it is possible to balance mountability andhigh-speed responsiveness in a simple manner.

3. Second Embodiment

[Configuration]

Next, a configuration of a light receiving element 2 according to asecond embodiment of the disclosure is described. FIG. 16 illustrates anexample of a cross-sectional configuration of the light receivingelement 2 according to the present embodiment. The light receivingelement 2 is preferably applicable to an optical communication devicethat performs signal transmission (optical transmission) betweensemiconductor chips such as LSIs. The light receiving element 2 is alight receiving element of a rear surface incident type, and includes,for example, a semiconductor layer 50. The semiconductor layer 50 has adevice structure in which, for example, a second conductive layer 51, anoptical absorption layer 52, and a first conductive layer 53 arelaminated in this order, starting from a side of a light incidentsurface 50A.

The second conductive layer 51 (specifically, a lower portion of thesecond conductive layer 51), the optical absorption layer 52, and thefirst conductive layer 53 constitute a mesa portion 55 having a pillarshape. For example, the pillar-shaped mesa portion 55 has a shape of acylinder that extends in a normal direction of the light incidentsurface 50A. The pillar-shaped mesa portion 55 may have a side surfacethat is parallel to the normal direction of the light incident surface50A, or that has a forward tapered shape or a reverse tapered shape. Anupper surface of the second conductive layer 51 (a surface, of thesecond conductive layer 51, located on an opposite side of the opticalabsorption layer 52) is the light incident surface 50A through whichlight enters from outside. A lower surface of the mesa portion 55includes a lower surface of the first conductive layer 53 (a surface, ofthe first conductive layer 53, located on an opposite side of theoptical absorption layer 52).

Each of the second conductive layer 51 and the first conductive layer 53includes a semiconductor material of a conductivity type different fromeach other. The second conductive layer 51 includes, for example, n-typeAlGaAs. As an n-type impurity included in the n-type GaAs, silicon (Si)or selenium (Se) is given, for example. The first conductive layer 53includes, for example, p-type AlGaAs. As the p-type impurity included inthe p-type AlGaAs, zinc (Zn), magnesium (Mg), and beryllium (Be) aregiven.

The optical absorption layer 52 absorbs the light entering the lightincident surface 50A, and converts the absorbed light into an electricalsignal (photocurrent) corresponding to an output level of the absorbedlight. For example, the optical absorption layer 52 includes undopedGaAs or undoped InGaAs. There is provided, in the pillar-shaped mesaportion 55 in the semiconductor layer 50, a photodiode having a PINstructure that includes the second conductive layer 51 (specifically,the lower portion of the second conductive layer 51), the opticalabsorption layer 52, and the first conductive layer 53. In other words,the light receiving element 2 includes a mesa-type PIN photodiode. It isto be noted that the electrical signal generated in the opticalabsorption layer 52 is inputted, as an optical communication signal,into an optical communication calculation circuit (not illustrated)coupled to a second electrode 61 and a first electrode 62 that aredescribed later. The optical communication signal is used in the opticalcommunication calculation circuit for determining a signal level of thelight that has entered the light incident surface 50A.

The light receiving element 2 further includes, for example, the secondelectrode 61 and the first electrode 62. The second electrode 61 iselectrically coupled to the second conductive layer 51 and is provided,for example, in contact with a rim of the mesa portion 55 (specifically,a flat surface of the second conductive layer 51). For example, thesecond electrode 61 has a configuration in which an alloy of gold (Au)and germanium (Ge), nickel (Ni), and gold (Au) are laminated in thisorder, starting from a side of the second conductive layer 51. The firstelectrode 62 is electrically coupled to the first conductive layer 53and is provided, for example, in contact with the lower surface of thefirst conductive layer 53. For example, the first electrode 62 has adisc shape and covers the lower surface of the first conductive layer53. The first electrode 62 includes, for example, titanium (Ti),platinum (Pt), and gold (Au) that are laminated in this order, startingfrom a side of the first conductive layer 53.

Meanwhile, the semiconductor layer 50 includes, in the vicinity of theinterface 53A between the first conductive layer 53 and the opticalabsorption layer 52, a constricted portion 56 that is the mostconstricted part of the first conductive layer 53. For example, theconstricted portion 56 is rotationally symmetrical with respect to acentral axis of the mesa portion 55 as a rotational center (a linesegment parallel to an optical axis of the light receiving element 2 andpassing through a center of the mesa portion 55). The constrictedportion 56 is provided along an outer edge of the mesa portion 55,avoiding at least a portion of a region opposing the light incidentsurface 50A. In other words, the constricted portion 56 is provided in amanner not to interrupt current path in the semiconductor layer 50.

The constricted portion 56 has a wedge shape tapering toward the centralaxis of the mesa portion 55, and the interface 53A has an edge exposedon the internal surface of the constricted portion 56. The interface 53Ahas a smaller area than the interface between the first conductive layer53 and the second conductive layer 51 when assuming that the constrictedportion 56 is not provided. The narrowest part of the constrictedportion 56 has a depth (length) of, for example, not less than 2 μm andnot more than 20 μm. In a case where the constricted portion 56 has alength smaller than 2 μm, an effect produced by reducing the area of theinterface 53A (effect of reducing parasitic capacitance) is very small.In a case where the constricted portion 56 has a length larger than 20μm, a current path in the semiconductor layer 50 for photoelectronsgenerated in the vicinity of the side surface of the mesa portion 55within the optical absorption layer 52 is significantly larger than acurrent path in the semiconductor layer 50 for photoelectrons generatedin the middle of the mesa portion 55 within the optical absorption layer52. This is likely to have an adverse influence on high-speedresponsiveness.

FIG. 17 is an enlarged view illustrating an example of a cross-sectionalconfiguration in the vicinity of the constricted portion 56. The firstconductive layer 53 includes a constriction-formed layer 54 at thenarrowest part of the constricted portion 56. The constriction-formedlayer 54 includes a material having a relatively high etching ratecompared to another part of the first conductive layer 53 except for theconstricted portion 56. In a case where the first conductive layer 53includes n-type AlGaAs, the constriction-formed layer 54 includes, forexample, n-type AlAs or n-type AlGaInP. When using, as the etchant, thesolution in which phosphoric acid, hydrogen peroxide, and water aremixed at a ratio 3:1:50, AlAs has a higher etching rate than AlGaAs andGaAs. In addition, when using the mixed solution of hydrochloric acidand water as the etchant, AlGaInP has a higher etching rate than AlGaAsand GaAs. In a case where the first conductive layer 53 has a thicknessof about 2 μm and the optical absorption layer 52 has a thickness ofabout 2 μm, the constriction-formed layer 54 has a thickness of not lessthan 10 nm and not more than 100 nm, for example, and a thicknessbetween the interface 53A and the constriction-formed layer 54 withinthe first conductive layer 53 is about 100 nm, for example.

The light receiving element 2 further includes, for example, insulativemembers (buried layer 27 and insulating film 28) that protect theconstricted portion 26. The buried layer 27 is provided to bury theconstricted portion 26, and includes, for example, a resin material suchas polyimide. The insulating film 28 is provided to cover a surface ofthe buried layer 27 and a surface of the mesa portion 25, and includesan insulative material. As the insulative material used for theinsulating film 28, SiO2 and SiN are given, for example.

[Manufacturing Method]

Next, a method for manufacturing the light receiving element 2 accordingto the present embodiment is described. FIG. 18 illustrates an exampleof a manufacturing process of the light receiving element 2. FIG. 19illustrates an example of a manufacturing process that follows FIG. 18.FIG. 20 illustrates an example of a manufacturing process that followsFIG. 19. FIG. 21 illustrates an example of a manufacturing process thatfollows FIG. 20.

To manufacture the light receiving element 2, for example, a compoundsemiconductor is collectively provided on a substrate 40 that includesGaAs, using an epitaxial crystal growth method such as the MOCVD method.At this time, for example, as a material for the compound semiconductor,methyl-based organometallic gas such as trimethylaluminum (TMAl),trimethylgallium (TMGa), trimethylindium (TMIn), and arshin (AsH3) areused. As a material for donor impurity, hydrogen selenide (H2Se) isused, for example. As a material for acceptor impurity, dimethylzinc(DMZn) is used, for example.

It is to be noted that in FIG. 18, a first conductive layer 53D includesthe same material as the first conductive layer 53. Aconstriction-formed layer 54D includes the same material as theconstriction-formed layer 54. An optical absorption layer 52D includesthe same material as the optical absorption layer 52. A secondconductive layer 51D includes the same material as the second conductivelayer 51.

First, as illustrated in FIG. 18, for example, the second conductivelayer 51D (n-type AlGaAs), the optical absorption layer 52D (non-dopedGaAs), and the first conductive layer 53D (p-type AlGaAs) are providedon the substrate 40 in this order, starting from a side of the substrate40. At the same time, the constriction-formed layer 54D (n-type AlAs orn-type AlGaInP) is provided in the first conductive layer 53D. Next, forexample, a resist layer having a circular shape (not illustrated) isprovided, and thereafter the semiconductor layer that includes the firstconductive layer 53D, the constriction-formed layer 54D, the opticalabsorption layer 52D, and the second conductive layer 51D is etched intoa mesa shape, using the resist layer as a mask. At this time,wet-etching is used. Here, in a case where the constriction-formed layer54D includes n-type AlAs, the solution in which phosphoric acid,hydrogen peroxide, and water are mixed at a ratio 3:1:50 is used as theetchant. In a case where the constriction-formed layer 54D includesn-type AlGaInP, the solution in which hydrochloric acid and water ismixed is used as the etchant. Thus, as illustrated in FIG. 19, selectiveetching of the constriction-formed layer 54D forms, in the vicinity ofthe interface 21A between the first conductive layer 53 and the opticalabsorption layer 52 in the mesa (mesa portion 55), the constrictedportion 56 that is the most constricted part of the first conductivelayer 53, while exposing an edge of the interface 53A to the internalsurface of the constricted portion 56. As a result, the mesa portion 55including the constricted portion 56 in the side surface is provided.

Next, as illustrated in FIG. 20, for example, a buried layer 57 thatprotects the constricted portion 56 is provided. Subsequently, asillustrated in FIG. 21, for example, the insulating film 58, the firstelectrode 62, and the second electrode 61 are provided. Finally, forexample, the substrate 40 is polished and thereby removed. In thismanner, the light receiving element 2 according to the presentembodiment is manufactured.

[Effects]

Next, effects of the light receiving element 2 according to the presentembodiment are described.

In the present embodiment, the mesa portion 55 includes, in the vicinityof the interface 53A between the first conductive layer 53 and theoptical absorption layer 52, the constricted portion 56 that is the mostconstricted part of the first conductive layer 53 and the interface 53Ahas an edge exposed on the internal surface of the constricted portion56. This allows for a smaller parasitic capacitance of the element ascompared to the case of the mesa portion 55 without the constrictedportion 56. In addition, even in the case of a larger mesa diameter, itis possible to suppress an increase in the parasitic capacitance of theelement. Thus, it is possible to balance mountability and high-speedresponsiveness.

In addition, in the present embodiment, the constriction-formed layer 54is provided at the narrowest part of the constricted portion 56. Theconstriction-formed layer 54 includes a material having a relativelyhigh etching rate compared to another part of the first conductive layer53 except for the constricted portion 56. This makes it possible toeasily provide the constricted portion 56, utilizing selective etchingby wet etching. Thus, it is possible to balance mountability andhigh-speed responsiveness in a simple manner.

In addition, in the present embodiment, the narrowest part of theconstricted portion 56 has a depth (length) of, for example, not lessthan 2 μm and not more than 20 μm. This makes it possible to balancemountability and high-speed responsiveness.

In addition, in the present embodiment, the insulative member (buriedlayer 57 or insulating film 58) is provided that protects theconstricted portion 56. This makes it possible to suppress deteriorationin reliability as a result of providing the constricted portion 56.

4. Modification Examples of the Second Embodiment

Next, modification examples of the light receiving element 2 accordingto the second embodiment are described.

[Modification Example F]

In the second embodiment, as illustrated in FIG. 22, for example, theconstriction-formed layer 54 may be omitted, and the constricted portion56 may be provided in the vicinity of the interface 53A in thesemiconductor layer 50. However, in such a case, for example, theconstricted portion 56 may be provided in the following manner. Forexample, in the manufacturing process, a mask having an opening isprovided in a region where the constricted portion 56 is to be providedin a mesa-etched side surface of the first conductive layer 53 and theoptical absorption layer 52, and thereafter the constricted portion 56is formed by selectively etching the first conductive layer 53 and theoptical absorption layer 52 via the opening in the mask. In the presentmodification example, as with the foregoing embodiment, it is alsopossible to balance mountability and high-speed responsiveness.

[Modification Example G]

In the second embodiment and modification examples thereof, theconstriction-formed layer 54 may include more than one layer. Forexample, as illustrated in FIG. 23, the constriction-formed layer 54 maybe a laminate including two constriction-formed layers 54A and 54B. Atthis time, the constriction-formed layer 54A is provided closer to thefirst electrode 62, and the constriction-formed layer 54B is providedcloser to the optical absorption layer 52. For the same etchant, theconstriction-formed layer 54B has a higher etching rate than theconstriction-formed layer 54A. Thus, when the constriction-formed layers54A and 54B are etched using the same etchant, the constriction-formedlayer 54B is etched faster than the constriction-formed layer 54A, thusthe narrowest part of the constricted portion 26 is provided closer tothe optical absorption layer 23 in the constriction-formed layer 54B. Asa result, this makes it possible to easily provide the edge of theinterface 53A at a position closer to the center of the light receivingelement 2. Thus, it is not only possible to balance mountability andhigh-speed responsiveness, but also to easily achieve high-speedresponsiveness.

[Modification Example H]

In any of the second embodiment and modification examples thereof, theoptical absorption layer 52 may be omitted. At this time, for example,as illustrated in FIG. 24, there is provided, in the pillar-shaped mesaportion 55 in the semiconductor layer 50, a photodiode having a PNstructure that includes the second conductive layer 51 (specifically,the lower portion of the second conductive layer 51) and the firstconductive layer 53. In other words, in the present modificationexample, the light receiving element 2 includes a mesa-type PNphotodiode.

In the present modification example, the semiconductor layer 50includes, in the vicinity of the interface 53A between the firstconductive layer 53 and the second conductive layer 51, the constrictedportion 56 that is the most constricted part of the first conductivelayer 53. The constricted portion 56 has a wedge shape tapering towardthe central axis of the mesa portion 55, and the interface 53A has anedge exposed on the internal surface of the constricted portion 56. Theinterface 53A has a smaller area than the interface between the firstconductive layer 53 and the second conductive layer 51 when assumingthat the constricted portion 56 is not provided. The narrowest part ofthe constricted portion 56 has a depth (length) of, for example, notless than 2 μm and not more than 20 μm. In a case where the constrictedportion 56 has a length smaller than 2 μm, an effect produced byreducing the area of the interface 53A is very small. In a case wherethe constricted portion 56 has a length larger than 20 μm, within adepletion region that is provided in a region including the interface53A between the first conductive layer 53 and the second conductivelayer 51, a current path in the semiconductor layer 50 forphotoelectrons generated in the vicinity of the side surface of the mesaportion 55 is significantly longer than a current path in thesemiconductor layer 50 for photoelectrons generated in the middle of themesa portion 55 within the depletion region. This is likely to have anadverse influence on high-speed responsiveness. It is to be noted thatthe method for manufacturing the light receiving element 2 according tothe present modification example conforms to the method formanufacturing the light receiving element 1 according to the foregoingModification Example C.

In the present modification example, as with the foregoing secondembodiment, there is provided, in the vicinity of the interface 53Abetween the first conductive layer 53 and the second conductive layer51, the constricted portion 56 that is the most constricted part of thefirst conductive layer 53. This makes it possible to balancemountability and high-speed responsiveness.

In addition, in the present modification example, theconstriction-formed layer 54 is provided at the narrowest part of theconstricted portion 56. The constriction-formed layer 54 includes amaterial having a relatively high etching rate compared to another partof the first conductive layer 53 except for the constricted portion 56.This makes it possible to easily provide the constricted portion 56,utilizing selective etching by wet etching. Thus, it is possible tobalance mountability and high-speed responsiveness in a simple manner.

In addition, in the present modification example, the narrowest part ofthe constricted portion 56 has a depth (length) of, for example, notless than 2 μm and not more than 20 μm. This makes it possible tobalance mountability and high-speed responsiveness.

In addition, in the present modification example, the insulative member(buried layer 57 or insulating film 58) is provided that protects theconstricted portion 56. This makes it possible to suppress deteriorationin reliability as a result of providing the constricted portion 56.

[Modification Example I]

In any of the second embodiment and modification examples thereof, thesubstrate 40 may include an InP substrate. In this case, the secondconductive layer 51 includes, for example, n-type InGaAsP, and the firstconductive layer 53 includes, for example, p-type InGaAsP. In a casewhere the optical absorption layer 52 is provided, the opticalabsorption layer 52 includes, for example, non-doped InGaAs. In thiscase, the constriction-formed layer 54 includes, for example, n-typeInP. When using the mixed solution of phosphoric acid and hydrochloricacid as the etchant, InP has a higher etching rate than InGaAsP andGaAsP.

In the present modification example, the constriction-formed layer 54 isprovided at the narrowest part of the constricted portion 56. Theconstriction-formed layer 54 includes a material having a relativelyhigh etching rate compared to another part of the first conductive layer53 except for the constricted portion 56. This makes it possible toeasily provide the constricted portion 56, utilizing selective etchingby wet etching. Thus, it is possible to balance mountability andhigh-speed responsiveness in a simple manner.

[Modification Example J]

In any of the second embodiment and Modification Examples F to H, thesubstrate 40 may include an InP substrate. In this case, the secondconductive layer 51 includes, for example, n-type InP, and the firstconductive layer 53 includes, for example, p-type InP. In a case wherethe optical absorption layer 52 is provided, the optical absorptionlayer 52 includes, for example, non-doped InGaAsP. In this case, theconstriction-formed layer 54 includes, for example, n-type InAlAs. Whenusing the mixed solution of sulfuric acid, hydrogen peroxide, and wateras the etchant, InAlAs has a higher etching rate than InP and InGaAsP.

In the present modification example, the constriction-formed layer 54 isprovided at the narrowest part of the constricted portion 56. Theconstriction-formed layer 54 includes a material having a relativelyhigh etching rate compared to another part of the first conductive layer53 except for the constricted portion 56. This makes it possible toeasily provide the constricted portion 56, utilizing selective etchingby wet etching. Thus, it is possible to balance mountability andhigh-speed responsiveness in a simple manner.

5. Third Embodiment

[Configuration]

Next, an optical communication device 3 according to a third embodimentof the disclosure is described. FIG. 25 illustrates an example of across-sectional configuration of the optical communication device 3according to the present embodiment. The optical communication device 3includes two LSI chips 72 and 73 mounted on a printed wiring board 71.On a surface of one of the LSI chips, i.e, the LSI chip 72, a lightemitting element 74 such as a semiconductor laser is provided. The lightemitting element 74 converts an electrical signal from the LSI chip 72into an optical signal, and outputs the optical signal. On a surface ofthe other LSI chip 73, the light receiving elements 1 and 2 according toany of the foregoing embodiments and modification examples thereof areprovided. Each of the light receiving elements 1 and 2 converts, into anelectrical signal, the optical signal inputted into each of the lightreceiving elements 1 and 2, and inputs the electrical signal into theLSI chip 73.

A lens 75 is provided on a light emitting surface of the light emittingelement 74, each of the light incident surfaces 20A and 50A of therespective light receiving elements 1 and 2, and each end of an opticalwaveguide 78. For example, this lens 75 is a collimating lens thatcollimates divergent light and converges parallel light. In addition,there is provided, on an upper surface of each of the LSI chips 72 and73, a male connector 76 having a cylindrical shape and covering thelight emitting element 74 and the light receiving elements 1 and 2. Anupper surface of the male connector 76 has an opening 76A, with a femaleconnector 77 being provided to cover this opening 76A while being fittedin the male connector 76. This female connector 77 is provided along theoptical waveguide 78 and also serves to support the optical waveguide78.

In the present embodiment, when the light emitting element 74 is drivenafter the male connector 76 and the female connector 77 are coupled toeach other, the light emitting element 74 emits light, and the lightenters an end of the optical waveguide 78 via the lens 75. The lightthat has entered the optical waveguide 78 is guided through the opticalwaveguide 78 to be outputted from another end of the optical waveguide78, and enters the light receiving elements 1 and 2 via the lens 75. Thelight that has entered the light receiving elements 1 and 2 is convertedinto an electrical signal (photocurrent) corresponding to an outputlevel of the entered light, and then the electrical signal is outputtedto the LSI chip 73.

Meanwhile, in the present embodiment, the light receiving elements 1 and2 according to any of the foregoing embodiments and modificationexamples thereof are used in the optical communication device 3. Thisfacilitates mounting of the light receiving elements 1 and 2, thusmaking it possible to manufacture the optical communication device 3 atlow cost. In addition, each of the light receiving elements 1 and 2 hashigh-speed responsiveness, thus making it possible to perform opticalcommunication at high speed.

In the foregoing third embodiment, the optical communication device 3may include more than one light emitting element 74. In addition, in theforegoing third embodiment, the optical communication device 3 mayinclude more than one light emitting element 74. The first opticalcommunication device according to an embodiment of the disclosure mayinclude more than one light receiving element 1 or more than one lightreceiving element 2.

Although the disclosure has been described above referring to someembodiments and modification examples thereof, the disclosure is notlimited to any of the embodiments, and may be modified in a variety ofways. It is to be noted that effects described herein are merelyillustrative. Effects described herein are not limited by the effectsdescribed herein. Effects described herein may have other effects thanthe effects described herein.

In addition, for example, the disclosure may have the followingconfigurations.

(1)

-   A light receiving element including a semiconductor layer in which a    photodiode is provided in a mesa portion having a pillar shape, the    photodiode having a PIN structure that includes a first conductive    layer, an optical absorption layer, and a second conductive layer    having a light incident surface,-   the semiconductor layer including, in a vicinity of an interface    between the first conductive layer and the optical absorption layer,    a constricted portion that is a most constricted part of the first    conductive layer,-   the interface having an edge exposed on an internal surface of the    constricted portion.    (2)-   The light receiving element according to (1), in which-   the first conductive layer includes a constriction-formed layer at a    narrowest part of the constricted portion, the constriction-formed    layer including a material having a relatively high etching rate    compared to another part of the first conductive layer except for    the constricted portion.    (3)-   The light receiving element according to (1) or (2), in which-   the narrowest part of the constricted portion has a diameter smaller    than a light reception diameter of the light incident surface.    (4)-   The light receiving element according to any one of (1) to (3), in    which-   the narrowest part of the constricted portion has a depth of not    less than 2 μm and not more than 20 μm.    (5)-   The light receiving element according to any one of (1) to (4),    further including an insulative member that protects the constricted    portion.    (6)-   A light receiving element including a semiconductor layer in which a    photodiode is provided in a mesa portion having a pillar shape, the    photodiode having a PN structure that includes a first conductive    layer and a second conductive layer having a light incident surface,-   the semiconductor layer including, in a vicinity of an interface    between the first conductive layer and the second conductive layer,    a constricted portion that is a most constricted part of the first    conductive layer,-   the interface having an edge exposed on an internal surface of the    constricted portion.    (7)-   The light receiving element according to (6), in which-   the first conductive layer includes a constriction-formed layer at a    narrowest part of the constricted portion, the constriction-formed    layer including a material having a relatively high etching rate    compared to another part of the first conductive layer except for    the constricted portion.    (8)-   The light receiving element according to (6) or (7), in which-   the narrowest part of the constricted portion has a diameter smaller    than a light reception diameter of the light incident surface.    (9)-   The light receiving element according to any one of (6) to (8), in    which-   the narrowest part of the constricted portion has a depth of not    less than 2 μm and not more than 20 μm.    (10)-   The light receiving element according to any one of (6) to (9),    further including an insulative member that protects the constricted    portion.    (11)-   An optical communication device including one or more light    receiving elements,-   the one or more light receiving elements each including a    semiconductor layer in which a photodiode is provided in a mesa    portion having a pillar shape, the photodiode having a PIN structure    that includes a first conductive layer, an optical absorption layer,    and a second conductive layer having a light incident surface,-   the semiconductor layer including, in a vicinity of an interface    between the first conductive layer and the optical absorption layer,    a constricted portion that is a most constricted part of the first    conductive layer,-   the interface having an edge exposed on an internal surface of the    constricted portion.    (12)-   An optical communication device including one or more light    receiving elements,-   the one or more light receiving elements each including a    semiconductor layer in which a photodiode is provided in a mesa    portion having a pillar shape, the photodiode having a PN structure    that includes a first conductive layer and a second conductive layer    having a light incident surface,-   the semiconductor layer including, in a vicinity of an interface    between the first conductive layer and the second conductive layer,    a constricted portion that is a most constricted part of the first    conductive layer,-   the interface having an edge exposed on an internal surface of the    constricted portion.    (13)-   A method for manufacturing a light receiving element, the method    including an etching process that includes:-   etching a semiconductor layer into a shape of a mesa, the    semiconductor layer including a photodiode having a PIN structure    that includes a first conductive layer, an optical absorption layer,    and a second conductive layer;-   providing, in a vicinity of an interface between the first    conductive layer and the optical absorption layer in the mesa, a    constricted portion that is a most constricted part of the first    conductive layer; and-   exposing an edge of the interface to an internal surface of the    constricted portion.    (14)-   The method for manufacturing a light receiving element according to    (13), in which-   the first conductive layer includes a constriction-formed layer in    the vicinity of the interface in the first conductive layer, the    constriction-formed layer including a material having a relatively    high etching rate compared to another part of the first conductive    layer except for the constricted portion, and-   the constricted portion is formed by selectively etching the    constriction-formed layer in the etching process.    (15)-   A method for manufacturing a light receiving element, the method    including an etching process that includes:-   etching a semiconductor layer into a shape of a mesa, the    semiconductor layer including a photodiode, the photodiode having a    PN structure that includes a first conductive layer and a second    conductive layer;-   providing, in a vicinity of an interface between the first    conductive layer and the second conductive layer in the mesa, a    constricted portion that is a most constricted part of the first    conductive layer; and-   exposing an edge of an interface to the internal surface of the    constricted portion.    (16)-   The method for manufacturing a light receiving element according to    (15), in which-   the first conductive layer includes a constriction-formed layer in    the vicinity of the interface in the first conductive layer, the    constriction-formed layer including a material having a relatively    high etching rate compared to another part of the first conductive    layer except for the constricted portion, and-   the constricted portion is formed by selectively etching the    constriction-formed layer in the etching process.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

The invention claimed is:
 1. A light receiving element comprising asemiconductor layer in which a photodiode is provided in a mesa portionhaving a pillar shape, the photodiode having a PIN structure thatincludes a first conductive layer, an optical absorption layer, and asecond conductive layer having a light incident surface, thesemiconductor layer including, in a vicinity of an interface between thefirst conductive layer and the optical absorption layer, a constrictedportion that is a most constricted part of the first conductive layer,the interface having an edge exposed on an internal surface of theconstricted portion, wherein a narrowest part of the constricted portionhas a depth from 2 μm to 20 μm.
 2. The light receiving element accordingto claim 1, wherein the first conductive layer includes aconstriction-formed layer at a narrowest part of the constrictedportion, the constriction-formed layer including a material having arelatively high etching rate compared to another part of the firstconductive layer except for the constricted portion.
 3. The lightreceiving element according to claim 1, wherein the narrowest part ofthe constricted portion has a diameter smaller than a light receptiondiameter of the light incident surface.
 4. The light receiving elementaccording to claim 1, further comprising an insulative member thatprotects the constricted portion.
 5. A light receiving elementcomprising a semiconductor layer in which a photodiode is provided in amesa portion having a pillar shape, the photodiode having a PN structurethat includes a first conductive layer and a second conductive layerhaving a light incident surface, the semiconductor layer including, in avicinity of an interface between the first conductive layer and thesecond conductive layer, a constricted portion that is a mostconstricted part of the first conductive layer, the interface having anedge exposed on an internal surface of the constricted portion, whereina narrowest part of the constricted portion has a depth from 2 μm to 20μm.
 6. The light receiving element according to claim 5, wherein thefirst conductive layer includes a constriction-formed layer at anarrowest part of the constricted portion, the constriction-formed layerincluding a material having a relatively high etching rate compared toanother part of the first conductive layer except for the constrictedportion.
 7. The light receiving element according to claim 5, whereinthe narrowest part of the constricted portion has a diameter smallerthan a light reception diameter of the light incident surface.
 8. Thelight receiving element according to claim 5, further comprising aninsulative member that protects the constricted portion.
 9. An opticalcommunication device comprising one or more light receiving elements,the one or more light receiving elements each including a semiconductorlayer in which a photodiode is provided in a mesa portion having apillar shape, the photodiode having a PIN structure that includes afirst conductive layer, an optical absorption layer, and a secondconductive layer having a light incident surface, the semiconductorlayer including, in a vicinity of an interface between the firstconductive layer and the optical absorption layer, a constricted portionthat is a most constricted part of the first conductive layer, theinterface having an edge exposed on an internal surface of theconstricted portion, wherein a narrowest part of the constricted portionhas a depth from 2 μm to 20 μm.
 10. An optical communication devicecomprising one or more light receiving elements, the one or more lightreceiving elements each including a semiconductor layer in which aphotodiode is provided in a mesa portion having a pillar shape, thephotodiode having a PN structure that includes a first conductive layerand a second conductive layer having a light incident surface, thesemiconductor layer including, in a vicinity of an interface between thefirst conductive layer and the second conductive layer, a constrictedportion that is a most constricted part of the first conductive layer,the interface having an edge exposed on an internal surface of theconstricted portion, wherein a narrowest part of the constricted portionhas a depth from 2 μm to 20 μm.
 11. A method for manufacturing a lightreceiving element, the method comprising an etching process thatincludes: etching a semiconductor layer into a shape of a mesa, thesemiconductor layer including a photodiode having a PIN structure thatincludes a first conductive layer, an optical absorption layer, and asecond conductive layer; providing, in a vicinity of an interfacebetween the first conductive layer and the optical absorption layer inthe mesa, a constricted portion that is a most constricted part of thefirst conductive layer; and exposing an edge of the interface to aninternal surface of the constricted portion, wherein a narrowest part ofthe constricted portion has a depth from 2 μm to 20 μm.
 12. The methodfor manufacturing a light receiving element according to claim 11,wherein, the first conductive layer includes a constriction-formed layerin the vicinity of the interface in the first conductive layer, theconstriction-formed layer including a material having a relatively highetching rate compared to another part of the first conductive layerexcept for the constricted portion, and the constricted portion isformed by selectively etching the constriction-formed layer in theetching process.
 13. A method for manufacturing a light receivingelement, the method comprising an etching process that includes: etchinga semiconductor layer into a shape of a mesa, the semiconductor layerincluding a photodiode, the photodiode having a PN structure thatincludes a first conductive layer and a second conductive layer;providing, in a vicinity of an interface between the first conductivelayer and the second conductive layer in the mesa, a constricted portionthat is a most constricted part of the first conductive layer; andexposing an edge of the interface to an internal surface of theconstricted portion, wherein a narrowest part of the constricted portionhas a depth from 2 μm to 20 μm.
 14. The method for manufacturing a lightreceiving element according to claim 13, wherein the first conductivelayer includes a constriction-formed layer in the vicinity of theinterface in the first conductive layer, the constriction-formed layerincluding a material having a relatively high etching rate compared toanother part of the first conductive layer except for the constrictedportion, and the constricted portion is formed by selectively etchingthe constriction-formed layer in the etching process.